Metals are required nutrients for animals, including humans and domesticated livestock. The U.S. Dept. of Agriculture has reported the following minimal Recommended Daily Allowances (RDAs) for selected metals in human diets: chromium (0.20 mg), iron (10 mg), manganese (2 mg), selenium (0.07-0.20 mg) and zinc (15 mg). Requirements for specific metals are revealed by the health problems associated with diets deficient in those particular metals. One well-known example is iron deficiency anemia, associated with inadequate levels of iron. With respect to selenium, inadequate levels have been implicated in human cardiovascular diseases, such as Keshan disease, as well as cancer. Chromium is required for insulin activity. Not surprisingly, a lack of chromium interferes with fat, carbohydrate and protein metabolism, leading to a variety of adverse conditions in humans. With respect to zinc, Sandstead, Am. J. Dis. Child. 145:853-859 (1991) reported that a deficiency in humans leads to a suppressed immune system, poor wound healing, dermatitis, and pregnancy complications. And as a final example, manganese deficiency interferes with connective tissue development in animals, leading to structurally defective tissues. Beyond these specific examples, several other metals are needed by humans and other animals to maintain health.
Metal deficiencies, and the resulting health problems for humans and other animals, are likely to be widespread. For example, Sandstead (1991) state that mild zinc deficiencies are common in some human populations. Perhaps more strikingly, chromium deficiency in the well-fed U.S. population has been reported to reach 90% of the population. Efforts to redress metal deficiencies have led to the inclusion of metals in dietary supplements, e.g. trace mineral tablets and capsules, which are costly and typically provide the metal in inorganic form. The form of a metal is significant in that it affects the bioavailability of the metal. Metal bioavailability in a nutritional sense is dependent on the solubility of the metal and the absorbance of the metal by the consuming organism. Generally, a more soluble form of a metal is more readily bioavailable. Unlike the metal forms found inside living organisms, simple inorganic forms of metals exhibit a relatively low solubility and, hence, bioavailability. This, in turn, leads to costly incorporation of excessive quantities of inorganic metal forms in most mineral tablets and capsules as a means of providing even trace quantities of metal for nutritional purposes.
Metals provided as constituents of plants are frequently more palatable and bioavailable than the simple inorganic forms of these metals. For example, metals within plants may be coordinated in compounds that facilitate assimilation by simply increasing the effective solubility of the metal. Provision of nutritionally significant quantities of metals in the form of plant producers is severely hindered by the fact that conventional agricultural methods result in plants with metal levels that are too low to be of practical use as nutritional supplements. Moreover, absent significant control of environmental factors throughout the period of plant growth, there frequently exists a prospect that unwanted or even toxic metals might also be incorporated in plants along with desired metals.
A variety of mature plants have been analyzed for metal content, with the following results (metal concentration in mg/kg dry plant matter noted parenthetically): boron (15-100), chromium (&lt;10), cobalt (0.2-29), copper (5-15), iron (18-1,510), magnesium (2,500-10,000), manganese (5-1,840), molybdenum (0.5-5), selenium (1,355 in garlic, 1,922 in yeast), and zinc (1.2-75). See, Kabata-Pendias et al, in Trace Elements in Soils and Plants, at pages 108 (Table 59), 230 (Table 142), and 238 (Table 146) (CRC Press Boca Raton Fla. 1989); Robb et al., in Metals and Micronutrients: Uptake and Utilization by Plants, at page 5 (Table I) (Academic Press NY 1983). At these levels, adequate metal uptake either cannot be attained or is achieved only with the impractical devotion of a substantial portion of the diet to metal-containing plant foodstuffs. Moreover, supplementing diets with metals from plants introduces the expense and delay of plant cultivation. Also, significant local variation in the metal content of conventional growth environments such as soil has led to unreproducible and, hence, unpredictable levels of metals in plants.
Modifications in plant cultivation methods, such as contacting plants with solutions containing high metal concentrations, have led, e.g., to elevated levels of selenium (i.e., Se) in higher plants such as garlic and brussel sprouts. To date these levels have not exceeded 1,382 mg/kg (i.e., ppm) of dry plant matter. Bird et al., J. Chroma. A 789:349-359 (1997); see also, Ip et al., Cancer Res. (Supp.) 54:1957s-1959s, 1994; Stoewsand et al., Cancer Lett. 45:43-48, 1989).
Of interest to the background of the invention is the use of plants to remove metal pollutants by "phytoremediation." Baker et al., Biorecovery 1:81-126 (1989); Banuelos et al., J. Environ Qual. 19:772:777 (1990); Banuelos et al., J. Environ. Qual. 22:786-792 (1993); Cunningham et al., Plant Physiol. 110:715-719 (1996). Illustratively, U.S. Pat. No. 5,364,451 discloses the use of genetically modified mature plants to remove metals from soils; U.S. Pat. No. 5,785,735 teaches plant-based metal removal from soils manipulated by the addition of chelating agents. In an analogous manner, for aqueous environments, U.S. Pat. No. 5,393,426 discloses the removal of metals using mature plants or portions of mature plants. U.S. Pat. Nos. 5,364,451, 5,785,735, and 5,393,426 are each incorporated herein by reference in their entireties.
Another form of phytoremediation is disclosed in U.S. Pat. No. 5,728,300, which is incorporated by reference herein in its entirety. Plants as young as 1-14 days old are shown to be capable of "depleting" contaminating, usually toxic, metals from aqueous solutions, with the capacity for depletion becoming more pronounced with increasing age. For seedlings less than eleven days old, metal accumulation levels, adjusted upward to account for any variation, did not exceed the following values (accumulation levels originally disclosed in terms of bioaccumulation coefficients have been converted herein to relative mass measures in terms of mg/kg): arsenic (1,400), chromium (1,600), cobalt (800), copper (5,600), magnesium (5,700), manganese (7,500), nickel (450), potassium (19,500), and zinc (3,600). For purposes of phytoremediation, of course, it is not necessary or even relevant to the process that the accumulated metals be processed by the plants into bioavailable forms. The plants will successfully perform their metal-depleting function whether the metals are accumulated in plant tissue or simply adsorbed as inorganic salts on the surfaces of plant roots.
Also of interest to the background of the invention are the disclosures of co-owned, co-pending U.S. patent application Ser. No. 09/041,355, incorporated herein by reference in its entirety, which discloses the use of mature plants having increased levels of accumulated metals as nutritional supplement constituents. Accumulated metals within the plants are noted to exhibit increased bioavailabilty, as determined using the serial solubilization method of Berti et al, Proc. Third Int'l. Conf Biogeochem. of Trace Elements (1995).
Production of metal-enriched mature plants, of course, is rather labor intensive and requires energy in the form of light, as well as nutrients, space, and time for the plants to grow and accumulate metals. The provision of such requirements often adversely impinges on the overall cost-effectiveness of the procedures. Thus, a need continues to exist in the art for new, cost-effective means for generating plant sources of metal nutrients that can reproducibly provide the increased quantities of bioavailable metals necessary to remedy health-impairing metal deficiencies of man and other animals.